There is no doubt that test prep, especially focused one on one tutoring with a good tutor, can provide some benefit for certain students, maybe even substantial benefit for a few. There is also no doubt that too much or inefficacious prep also hurts some students, relative to what they would have achieved had they undertaken it on their own. All kids are different, not all tutors are particularly good, and few parents can make an objective determination when it comes to their kids.
In the aggregate, I tend to believe the academic studies that on average test prep only has a small effect, on the order of 0.10 - 0.15 standard deviations.